Primary Source of Energy During a High Intensity Aerobic Activity

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STEVEN LOWIS Last Updated: Aug 20, 2015

Steven Lowis

Steven Lowis is a teacher of metaphysics, as well as a writer covering a wide range of topics. He specializes in the areas of quantum theory, physics, biology, health and fitness, psychology, theology and philosophy. He has released a book titled "The Meaning of Life - Understanding Purpose and the Nature of Reality."

Four women working hard in an aerobics class.Photo Credit dolgachov/iStock/Getty Images

Aerobic activity is any physical activity that uses the large muscles of your body continually and rhythmically at a constant level of intensity. Some examples of high-intensity aerobic activity include aerobic dance classes, swimming laps in a pool, running and sports like basketball or singles tennis. However, there’s a point where aerobic exercise become anaerobic exercise, and that’s when your body switches its primary source of energy.

The Fuel in Your Tank

Your body uses different sources of energy, depending on how hard you work. For low-intensity aerobic exercise, where you will reach anywhere from 35 to 65 percent of your heart rate maximum, your body will use fat stores as calories to burn for energy. Switching to a higher gear, when you’re engaging in any high-intensity activity (80 to 90 percent of your maximum heart rate), your body will turn to a molecule called creatine phosphate first, and then carbohydrates for energy. These carbohydrates are taken from the glycogen stored in your liver and muscles. To calculate your maximum heart rate, subtract your age from 220.

Fuel Gauge Reading Empty

Your body stores more fat than it does glycogen -- around 70,000 calories of fat compared to around 2,000 calories of glycogen. If you take part in too much high-intensity exercise, your body will run out of glycogen supplies faster than it would run out of fat supplies. As your muscles and brain use glycogen to work properly, you're more likely to become tired both physically and mentally from high-intensity physical activity.

Preparing for the Road

The best way to ensure you have plenty of energy for a high-intensity workout is to eat between 300 to 600 calories of mostly carbohydrates a few hours beforehand, with a little protein and some fat. According to the American College of Sports Medicine, the amount of carbohydrates to consume hits a ratio of 2 to 3 grams per kilogram of body weight. Foods such as oatmeal, fruits, nuts and bread should be considered, along with fat-free dairy products.

Refueling and Re-energizing

During your aerobic activity, you can top-up energy levels with energy drinks or fruits like bananas or oranges. The American College of Sports Medicine recommends 30 to 60 grams of carbohydrate per hour (120 to 240 calories) of high-intensity activity. Once you’re finished, refuel with a snack of between 300 to 400 calories, adding up to around 75 to 100 grams of carbohydrates and around 6 grams of protein.